Analog Circuit Design

Analog Circuit DesignAVI SINGH

AmplifierWhat is an amplifier?

Necessary Conditions There is a “power gain”.

Voltage Amplifier Input and output: both are voltages

LHS: Input Terminal

RHS: Output Terminal

The bottom wire is similar to a ‘common ground’

An equivalent Circuit Note: There is a voltage-dependent current source in the circuit.

How do we build an amplifier? Amplifiers cannot be built simply using resistors, capacitors or inductors alone.

You need external power source also ( for the power gain).

Some ‘simple’ amplifier designs It is not possible for everyone to design their own amplifiers without specialised knowledge.

So, how do we proceed?Our dilemma: Amplifiers already might not satisfy all our specific requirements, while it is very difficult to design a good amplifier from scratch.

The answer: A Semi-Custom Approach

We use some amplifiers that have been already built, and customize them according to our own requirements.

An Example of such an amplifier that we can customize according to our needs: An Operational Amplifier

Difference Amplifiers They are insensitive to what is common between two signals, and sensitive to what is different.

An Operational Amplifier(Op-Amp)

A special type of difference amplifier

Very high common mode rejection ratio.

Data sheet of a 741 op-amp

Equivalent Circuit of an Op-Amp

Feedback

Negative and Positive Feedback

Different Op-Amp Circuits We mostly use op-amp circuits that employ negative feedback.

Op Amp with Negative Feedback

Virtual Ground Property

Virtual Ground Property In op-amp circuits employing negative feedback, the voltage at the positive input terminal is almost equal to the voltage at the negative input terminal, given that the gain of the op-amp is very high.

It does not hold under:

1. Open Loop System

2. Positive Feedback

3. If the op-amp is saturated (that is, it is driving the maximum current that it can)

Golden Rules for Analysing Op-Amp Circuits

Note: These hold true only and only under negative feedback.

Some simple amplifiers made using Op-Amps

Inverting Amplifiers

Non-Inverting Amplifiers

An Adder

Integrator

Differentiator

Instrumentation Amplifier

Instrumentation Amplifier Has limited use: not as flexible as the operational amplifierMade up for four op-amps

Feedback resistor is internal

Mainly used for high-precision gain

Only one of the resistors is determined externally that is Rg.

Analysis

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